I read here and there that someone has beaten thus and such scenario or has beaten level whatever. Is this different than finished? Is there a specific set of conditions to be met in order to beat a scenario?

That is, you have to go from beginning to end

in one session without dying, orin one session but you can die and continue a saved game, orin several sessions without dying, but you can save a game, so you can go to school or to work, then continue it later, orjust simply go from beginning to end in as many sessions and dying as many times as it takes.

As an example, I went from beginning to end of Durandal on TC, but it took several months and I died many times, though less often as the game progressed. Did I beat the scenario; finish the scenario; or demonstrate that I don’t have a life?

Actually, I think that playing Marathon is as valid an activity as anything else people do for entertainment.

Specifically in the context of terminal writing, Finished means when the level's mission parameters have been satisfied, like collecting a repair chip. Otherwise, finished and beaten are synonymous. People refer to completing a Marathon level under certain stricter conditions as vidding it, as in, like a Vidmaster. That might be the term you're looking for.